Activation lost after adding differencing disk to an activated VM.

Continuing from http://social.technet.microsoft.com/Forums/en-US/winserverhyperv/thread/4961faf6-d7c3-456b-9564-ce4eeea2bc2b

This is all being done under Hyper-V, guest/host OS are both Win 2008 r2. 

Creating snap shots won't cause the VM to loose activation but adding a differencing disk gets the VM deactivated. This did not used to happen with Win 2k3.

Any tricks to over come this?

yup